In general, the rarer a decay, the more sensitive it is to the influence of new particles. Among the rarest is when a beauty quark annihilates with an oppositely charged antiquark to produce a muon and an antimuon. In the Standard Model of particle physics, this process occurs through a complicated set of interactions, making the decay extraordinarily unlikely: less than a few times per billion beauty particles.
However, in the zeptoverse, new particles could provide an additional route for the decay to follow, altering how often it occurs. For decades, these decays have been seen as ‘golden channels’ in the search for new physics. “Physicists have been looking for this rare type of decay since 1984, which was before I was born,” said Cliff.
In 2024, a major upgrade to the LHCb experiment made it possible that this rarest of decays – the Bd to mu mu decay – might be found at last. An enormous new sample of beauty decays from 2024 and 2025 harnessed the ability of the upgraded LHCb experiment to tell different types of particles apart. By using machine learning techniques, the researchers were able to suppress the huge amounts of background noise that would otherwise bury these very rare signals, allowing them to obtain the first evidence for the Bd to mu mu decay.
By combining the 2024 and 2025 data with earlier data collected between 2011 and 2018, the strength of the signal rose to 3.2 sigma, above the ‘three sigma’ level of statistical significance, the accepted evidence threshold for a new phenomenon.
“Seeing clear evidence for this rarest decay for the first time was thrilling,” said Cliff. “We’re knocking on the door of the zeptoverse now. I can’t wait to see what we find as we explore deeper.”
The new results agree with the predictions of the Standard Model, giving physicists information about which particles could be found when the LHC’s successor comes online in the second half of this century. Another major upgrade, planned for the middle of the 2030s, could increase the rate of data collection by another order of magnitude, turning these discoveries into precision probes to search for undiscovered particles, giving physicists new opportunities to find cracks in the Standard Model and reveal what may lie beyond.
Professor Tim Gershon from the University of Warwick, who leads the LHCb experiment, said : “This fantastic result showcases the extraordinary capabilities of LHCb to probe nature at the fundamental level. This project, driven and led by UK physicists, is producing beautiful science. My focus is now on delivering the upgrade of LHCb planned for the 2030s, which will be essential if we are to continue on this journey of discovery.”
The UK’s contribution to CERN is supported by the Science and Technology Facilities Council (STFC), part of UK Research and Innovation (UKRI).
Reference:
LHCb collaboration. ‘Evidence for the decay B0 → μ+μ− and improved measurements of B0s → μ+μ−.’